1. Field
The present invention relates generally to communications, and more specifically, to improving the feedback of channel information, which can be used to improve the scheduling and rate control of traffic over a wireless communication system.
2. Background
The field of wireless communications has many applications including, e.g., cordless telephones, paging, wireless local loops, personal digital assistants (PDAs), Internet telephony, and satellite communication systems. A particularly important application is cellular telephone systems for mobile subscribers. As used herein, the term “cellular” system encompasses both cellular and personal communication services (PCS) frequencies. Various over-the-air interfaces have been developed for such cellular telephone systems including, e.g., frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA). In connection therewith, various domestic and international standards have been established including, e.g., Advanced Mobile Phone Service (AMPS), Global System for Mobile (GSM), and Interim Standard 95 (IS-95). IS-95 and its derivatives, IS-95A, IS-95B, ANSI J-STD-008 (often referred to collectively herein as IS-95), and proposed high-data-rate systems are promulgated by the Telecommunication Industry Association (TIA) and other well known standards bodies.
Cellular telephone systems configured in accordance with the use of the IS-95 standard employ CDMA signal processing techniques to provide highly efficient and robust cellular telephone service. Exemplary cellular telephone systems configured substantially in accordance with the use of the IS-95 standard are described in U.S. Pat. Nos. 5,103,459 and 4,901,307, which are assigned to the assignee of the present invention and incorporated by reference herein. An exemplary system utilizing CDMA techniques is the cdma2000 ITU-R Radio Transmission Technology (RTT) Candidate Submission (referred to herein as cdma2000), issued by the TIA. The standard for cdma2000 is given in the draft versions of IS-2000 and has been approved by the TIA and 3GPP2. Another CDMA standard is the W-CDMA standard, as embodied in 3rd Generation Partnership Project “3GPP”, Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214.
The telecommunication standards cited above are examples of only some of the various communication systems that can be implemented. Some of these various communication systems are configured so that remote stations can transmit information regarding the quality of the transmission medium to a serving base station. This channel information can then be used by the serving base station to optimize the power levels, the transmission formats, and the timing of forward link transmissions, and further, to control the power levels of reverse link transmissions.
As used herein, “forward link” refers to the transmissions directed from a base station to a remote station and “reverse link” refers to transmissions directed from a remote station to a base station. The forward link and the reverse link are uncorrelated, meaning that observations of one do not facilitate the prediction of the other. However, for stationary and slow-moving remote stations, the characteristics of the forward link transmission path will be observed to be similar to the characteristics of the reverse link transmission path in a statistical sense.
Channel conditions of received forward link transmissions, such as the carrier-to-interference (C/I) ratio, can be observed by a remote station, which reports such information to a serving base station. The base station then uses this knowledge to schedule transmissions to the remote station selectively. For example, if the remote station reports the presence of a deep fade, the base station would refrain from scheduling a transmission until the fading condition passes. Alternatively, the base station may decide to schedule a transmission, but at a high transmission power level in order to compensate for the fading condition. Alternatively, the base station may decide to alter the data rate at which transmissions are sent, by transmitting data in formats that can carry more information bits. For example, if the channel conditions are bad, data can be transmitted in a transmission format with redundancies so that corrupted symbols are more likely to be recoverable. Hence, the data throughput is lower than if a transmission format without redundancies were used instead.
The base station can also use this channel information to balance the power levels of all the remote stations within operating range, so that reverse link transmissions arrive at the same power level. In CDMA-based systems, channelization between remote stations is produced by the use of pseudorandom codes, which allows a system to overlay multiple signals on the same frequency. Hence, reverse link power control is an essential operation of CDMA-based systems because excess transmission power emitted from one remote station could “drown out” transmissions of its neighbors.
In communication systems that use feedback mechanisms to determine the quality of the transmission media, channel conditions are continuously conveyed on the reverse link. A remote station monitors the channel quality of the forward link and feeds it back to the base station via the Reverse Channel Quality Indicator Channel (R-CQICH). The transmission of a channel quality value on the R-CQICH is carried out in every slot of the R-CQICH. For slow moving or stationary remote stations, the transmission of a channel quality value on each slot allows the base station to accurately predict the state of the forward link. However, when a remote station is traveling at a high velocity, the condition of the reverse link worsens so that the base station cannot accurately decode the received channel quality values within a designated frame error rate. Moreover, the high velocity causes fast fading conditions that the base station cannot accurately estimate using outdated channel quality values.